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Providing evidence to improve practice

Action: Rear and manage populations of solitary bees Bee Conservation

Key messages

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Several species of solitary bee in the family Megachilidae are reared and managed commercially as pollinators, mostly for the forage crop alfalfa, or fruit trees. These species readily nest in drilled wooden blocks, or stacked grooved boards of wood or polystyrene. Parasites and pathogens can be problematic and a number of control methods have been developed. Rearing methods have been investigated for two other species not yet commercially managed and one replicated trial shows that temperature regimes are important to survival. If rearing for conservation purposes is to be attempted, we would recommend a systematic review of these methods.

Three management trials with megachilids not commercially managed in the USA or Poland, and a review of studies of managed species, found that local populations can increase up to six-fold in one year under management if conditions are good and plentiful floral resources are provided.

Two replicated trials have reared solitary bees on artificial diets. One found high larval mortality in Osmia cornuta reared on artificial pollen-based diets, including honey bee-collected pollen. The other found Megachile rotundata could be reared on an artificial diet based on honey bee-collected pollen, but bees reared on synthetic pollen substitutes either died or had low pre-pupal weight.

 

Supporting evidence from individual studies

Rearing methods for solitary bees

1 

Bosch & Kemp (2002) review methods that have been developed for rearing three species of mason bee now used as orchard pollinators in Japan, USA and Europe respectively: Osmia cornifrons, O. lignaria, and O. cornuta. All three species will nest in holes drilled in wood or polystyrene, grooved wood or polystyrene boards stacked together, paper or cardboard tubes or reed stem sections. If nest cavities are too narrow or too short, more males will be reared. Temperature regimes are important to survival through four of the seven developmental stages identified: development (egg to adult, including dormant pre-pupal phase), pre-wintering, wintering and incubation prior to emergence. Responses to temperatures differ between species, and between populations from different areas within species. These should be experimentally studied to develop an effective rearing regime.

Releasing bees at a site in their nests (rather than as extracted cocoons) increases the chance of females nesting at the same site, but extracting cocoons is used to reduce the spread of fungal pathogens in the alfalfa leafcutter bee Megachile rotundata.

Exposure to predators and parasites can be reduced by: covering nest shelters with a screen (deters birds); releasing bees in excluder boxes to avoid re-use of old nests (fungus/mites); removing nest boxes after nesting activity (late-flying parasitoids); black light traps (kills Monodontomerus and other wasp enemies); acaricide and thermal shock treatment during dormant pre-pupa stage (reduces mite numbers); selective trapping of wasp predators (e.g. male Sapyga pumila wasps can be caught at night in 2.5 mm diameter cavities).

2 

One replicated trial in India examined methods of artificially rearing the subtropical leafcutter species Megachile flavipes, a species not commercially managed for pollination (Kapil & Sihag 1985). This species can be stored at the pre-pupal stage for 270 days. It showed least mortality (5-7%) under this treatment if stored at 12°C. Stored at 4°C, the lowest temperature in the study, mortality was 16-20%. The optimum incubation temperature following lower temperature storage was 28.5°C. The bees emerged over a shorter time period if stored at 8°C and incubated at 28.5°C.

3 

A trial with the sunflower leafcutter bee Megachile pugnata in Utah, USA, found that females will nest in drilled wooden nest blocks, preferring holes 15 cm deep (Parker & Frohlich 1985). They can be overwintered from November to June or July at 3°C in the laboratory, and emerge after incubation at 30°C. On release in a sunflower field, female bees of the species were recorded foraging and nesting in the field where they were released (released individuals not marked).

Can populations be augmented by rearing?

4 

A trial with Osmia sanrafaelae, native to the San Rafael Desert, Utah, USA, found that bees were induced to nest in pine wood nest boxes with drilled 9 mm holes inside a 6 ´ 6 ´ 2 m saran cloth cage placed over an alfalfa crop Medicago sativa (Parker 1985). Fifty males and 50 females were introduced to the cage in July, and although mortality in the nests was high (47%) the number of bees surviving to adulthood in the next generation was 4-fold higher than the number originally introduced (exact number not given).

5 

In a trial at two experimental farms near Poznan, western Poland from 1989 to 1994, the numbers of red mason bees Osmia rufa nesting in bundles of reed stem increased substantially year on year. Each winter, occupied reed stems were collected and healthy bee cocoons (not parasitized) were transferred to refrigerators and kept at 4°C over winter. These were placed out in incubators along with new nest boxes the following spring. At one site, the number of emerging bees increased from an originally introduced 1,453 bees in 1989 to 108,973 in 1994 (a 75-fold increase; Wójtowski et al. 1995). The number of emerging females each year was between 1.3 and 5.7 times the number of females the previous year. Based on these numbers, the density of red mason bees on the farm was estimated to have increased from 1 bee/ha to 1,353 female bees/ha or more over the six years. Bee numbers nesting at the second site followed a similar trajectory, but the experiment was ended after three years.

6 

A review of captive-rearing methods developed for orchard bees in the genus Osmia reports evidence that female populations have been increased by 2- to 3-fold for O. cornifrons and 5-fold for O. lignaria in orchards, in years with good weather and fruit tree flowering (Bosch & Kemp 2002). Poor weather during flowering or short blooming periods can lead to population losses (no experimental evidence reported).

7 

Captive-reared sunflower leafcutter bees Megachile pugnata were released into a 0.7 ha sunflower field in Utah, USA in 1982 (Parker & Frohlich 1985). A total of 186 females were released (not marked) and a maximum of 182 were subsequently counted resting in nesting blocks at night, in and around the field. Altogether, 690 nests were made across 90 nest blocks, and the surviving number of overwintering pupae when counted in October was 1,643.

Can solitary bees be reared on artificial diets?

8 

A replicated controlled laboratory experiment at the University of Bologna, Italy, found that 408 European orchard bees Osmia cornuta reared on pollen-based artificial diets showed high larval mortality (76-100%; Ladurner et al. 1999). The 331 control bees allowed to consume the pollen lump provided by their mother had lower mortality rates (4-32%). These authors suggest this is because female O. cornuta bees add something, perhaps an enzyme, to the pollen they provide for their larvae.

9 

Nelson et al. (1972) reared groups of up to 25 alfalfa leafcutter bees Megachile rotundata on two different pollen substitutes or honey bee-collected pollen, and compared them to larvae reared on pollen collected by the mother bee. Those reared on pollen substitutes either died (one type of synthetic diet) or had lower prepupal weights (average weights 15-40 mg) than the control group (55 mg). Bees reared on honey-bee collected pollen weighed more than the control group (average weights 64-71 mg).

Referenced papers

Please cite as:

Dicks, L.V., Showler, D.A. & Sutherland, W.J. (2010) Bee conservation: evidence for the effects of interventions. Pelagic Publishing, Exeter, UK